As bishop, St. Ambrose was immensely popular, and was well loved by his congregation. St. Ambrose was well known for his sermons, which were acclaimed to be "masterpiecs of Latin eloquence." It was in one of these sermons that Ambrose gained his most famous convert, St. Augustine.

Throughout his career, St. Ambrose fought a constant battle against pagans and heretics, particularly the Arians, using his influence on the emperor and the Roman senate to quell them.

A more thorough biography (from where much of the above comes from) is at britannica.com:
http://www.britannica.com/bcom/eb/article/4/0,5716,6144+1+6061,00.htmlThe Catholic Encyclopedia has an older, more complete biography at: http://www.newadvent.org/cathen/01383c.htm

Ambrose was born in Trier (then Augusta Treverorum), Gaul, sometime around the year 340. He died on the 4th of April, 397. His life would take place during the pivotal years in Roman history when the great empire was in decline, but had not yet fallen. Barbarians were encroaching on all sides, and in the capital, decadence reigned; yet the legions maintained, for the most part, a deadly military grip, and the emperors could still rule savagely and powerfully. The Christian Church was still in its infancy, and factions within it were vying for power. One struggle that Ambrose in particular would deal with was that between the Athenasians and the Arians.

In order to understand his life, one must also understand the environment in which he lived. His father, Ambrosius, held one of the highest positions available to non-royalty in the Roman Empire, ruling over France, Britain, Spain, and Tingitana in Africa. He grew up in some of the larger cities of the Empire, the youngest of three children. His sister, Marcellina, who become a nun, and his brother Satyrus, who would move to Milan to help his brother when he became bishop, were religious people; both also became saints. When his father died in 354, the family moved to Rome, and his widowed mother became very religious. Ambrose, too, became religious, but he also continued to learn several secular studies. For instance, he could speak and write Greek, which would become very important when he became a bishop. He then studied law and became so good at it that the Emperor appointed him governor of Milan. In this office he was nearly universally loved.

Meanwhile, in the bishopric of Milan, terrible things were happening to the Christians. After the year 355, when the Bishop Dionysius was dragged in chains to exile in the distant East, the seat was held by Auxentius, an Arian who violently hated the Christians. He was ignorant of Latin, famously a zealot, and capable of violently prosecuting as many Christians as he could. Finally, in 374 he died, ending a tyranny that had created injustice in the region for nearly twenty years. The bishops of the region, fearing another Arian in the position, petitioned the Emperor Valentinian to choose someone for the job. The Emperor declined, motioning for elections to be held. The people unanimously called for Ambrose, but he fled the job, explaining that he hadn't even been baptized. He even fled to Rome, to the house of a Senator, but the Emperor ordered the he return to Milan, and the Senator released him. Ambrose was baptized and on December 7, 374, he was ordained as the bishop.

During his time as bishop of Milan, he accomplished several great feats. Rather than involving himself in politics or writing, he concentrated most of his efforts on his flock. He used his church's golden treasures to ransom captives taken by the Visigoths during an invasion, making the point that material goods were nothing compared to human lives. When the Eastern Emperor Theodosius held Ambrose and his flock in the church during a siege, Ambrose taught the people to sing hymns so that they wouldn't dwell on their condition. Eventually, Theodosius asked to be let into the church to pray; Ambrose was not afraid to rebuke an emperor and told him that he stood beneath religion. This was nearly Roman heresy, but Theodosius himself was a deeply religious man and did not allowed Ambrose to criticize him. When the Emperor died, he had Ambrose by his side.

Ambrose also had a relationship with the Western Emperor. Late into Ambrose's reign, the Emperor died and a four-year old ascended the throne with his mother Justinia as regent. She was an Arian and strictly persecuted Christians; however, when a Roman general tried to lead a coup against Rome, she called upon Ambrose to speak to the general. His name was Maximus, and he was extremely ambitious, but Ambrose was able to stop the invasion. Immediately afterwards Justinia began persecuting Christians again, but when Maximus threatened again, Ambrose again went to speak to him at Justinia's request.

Ambrose died following the death of a young friend whom he had hoped to baptize. He was deeply saddened by the loss and sank into poor health. When he died, he left behind a legacy of forgiveness and kindness to all people, even Arians, and an example of a bishop who could chide emperors and rise above politics.